A total of 97 plant species composed of 82 genera under 40 families were recorded from Abohoy Gara Mountain (Supplemental material 2). The number of plant species documented was higher than similar alpine mountains in Ethiopia, Guassa Community Conservation Area (n=82) (Wodaj et al 2016), Guna Mountain (n=56) (Molla 2004), Sanettie plateau in Bale Mountains (n=65) (Gashaw and Fetene 1996) and Simien Mountain (n=86) (Melese et al 2017). Out of the total species recorded, 85 (87.63%) were herbs, Nine (9.28%) shrubs and three (3.09%) tree. The highest number of plant habit recorded was herbs, this finding is in line with the result of Afro-alpine vegetation of Guassa Community Conservation Area (Wodaj et al 2016). Asteraceae was the dominant plant families represented by 15 (18%) genera and 21 (21.6%) species followed by Poaceae composed of seven (8.53) genera and eight (8.24%) species (Supplemental material 3). The occurrence of greater number of plants species from family Asteraceae might relate to the adaptation potential of the species under this family in a wider range of elevation in the study area. This finding is also in line with the results of (Wodaj et al 2016) in that Asteraceae was the highest family recorded.
Records of endemic plants
We recorded 16 (16.5%) plant species belonging to 14 genera under nine families endemic to Ethiopia (Supplemental material 2). Our record of endemic plants was higher than the endemic plants recorded from Guassa community conservation area (Wodaj et al 2016). Family Asteraceae contributed seven endemic plant species, Poaceae contributed two endemic plant species and six other families contributed just one endemic plant species (Supplemental material 3). It should be stressed that, two near threatened, one vulnerable and one least concern endemic plant species were found in the IUCN Red list categories (Vivero at al 2006). Becium grandiflorum and Inula confertiflora were near threatened, Euryops pinifolius was vulnerable and Solanecio gigas was least concern (Vivero at al 2006) (Supplemental material 2). The reason for mountain ecosystems often possessing several endemic plant species is due to alpine ecosystems make island biota and the high mountain plateau protrude as isolated temperate islands above the hot nearby plains (Beniston 2006).
Plant community along elevational gradient
We identified five plant communities for the vegetation of Abohoy Gara Mountain from the output of Agglomerative Hierarchical Classification (Figure 2). The names for each community were given based on two species of a cluster that have the highest synoptic value (Table 1). The number of community identified for Abohoy Gara Mountain was higher than for Choke Mountain range (n=3) (Belachew 2015) and Simien Mountain (n=3) (Melese at al 2017).
I. Festuca abyssinica - Thymus schimperi community
This herbaceous community consisting of 14 plots and 50 species was situated at elevational range of 3529 – 3813 m a.s.l (Table 2). F. abyssinica and T. schimperi were the dominant plant species of the community. Others, such as Festuca macrophyla, Alchemilla abyssinica and Arabis alpina were also present in this community. Achyranthes aspera , Plantago lanceolata and Plantago palmata were the species recorded only in this community.
II. Erica arborea- Hypericum revolutum community
This community structure consisting of 10 plots and 42 species was distributed at elevations above sea level 3569 – 3697 m (Table 2). E. arborea and H. revolutum were the dominant plant species of the community. Most frequently, a mass of moss species (Neckera platyantha) grew on the ground and under the Erica shrub species (Supplemental material 2). This community occurred predominantly in swampy areas. Elevational distribution of E. arborea in this community is comparable to that of Choke Mountain (3450 – 3814 m a.s.l) (Belachew 2015).
III. Alchemilla abyssinica - Koeleria pyramidata community
This community structure consisting of 13 plots and 73 species was situated at elevational range between 3507 – 3657 m a.s.l (Table 2). It was found to cover the largest area of the Afro-alpine vegetation of the study area with large number of plant species. A. abyssinica and K. pyramidata were the dominant plant species of this community. Species such as Crotalaria karagwensis and Cyperus reduncus, Primula verticillata, Senra incana and Spergula arvensis were character species in that they were found particularly in this community.
IV. Euryops pinifolius - Cyathula cylindrical community
This community structure consisting of 10 plots and 27 species was distributed at elevational range between 3462 – 3914 m a.s.l (Table 2). E. pinifolius and C. cylindrical were the dominant plant species of the community. C. cylindrica is most dominantly found in the upper hill of the mountain from 3700 - 3914 m where there is lack of moisture and sloppy ground while E. pinifolius were most dominantly distributed at the lower and middle elevation and it becomes sparse at the higher elevation.
V. Lobelia rhynchopetalum - Kniphofia foliosa community
This community structure consisting of 6 plots and 38 species was distributed at elevational range between 3570 - 3657 m a.s.l (Table 2). L. rhynchopetalum and K. foliosa were the dominant plant species. This community has a small number of plots and species compared to the other communities. Helichrysum formosissimum and I. confertiflora were character species recorded only in this community type. In this community type there is very low temperature where during morning water stream is changed into ice.
The alpha diversity indices across elevations showed that species richness and evenness were maximum in the mid elevation (community III) and tend to decrease gradually toward the high and lower elevations (Table 2). However, they failed to show an obvious U-shaped curve typical of the ‘mid- elevation bulge theory’ predicting a higher species diversity at mid-elevations. However, the variations in species diversity (H) were not strongly obvious; this could be due to related environmental factor such as elevation, slope and aspect.
Similarities between community types
Sorenson’s similarity between community showed that community type I and III showed the highest plant species similarity (0.72) whereas community type III and IV showed the least plant species similarity (The highest dissimilarity) (0.524) (Table 3). The maximum similarity between community I and III could be due their related elevational range of the two communities (Table 2).
Influence of elevation, slope and aspect on species richness
Species richness was negatively correlated with elevation, and their correlation was significant (r = -0.471, P < 0.05) (Figure 3a). Besides, significant difference (P < 0.05) was occurred in the mean number of species richness along elevational gradient. The mean value of species richness for upper elevation (3703-3900m) was 13.88, for the middle elevation (3620-3697m) was 17.47, and for the lowest elevation (3462-3605m) was (21.79), indicating that as elevation increases, species richness gets lower in the study area. Similar result was reported from Abune Yosef Mountain (Gebrehiwot et al 2019) and Arsi Mountain (Girma et al 2018). Number of species was negatively correlated with slope however their correlation was insignificant (r = -0.156, P > 0.05) (Figure 3b). This finding is in line with the report of Wondo Genet Forest (Kebede et al 2013). Slope affects the run-off and drainage through that way also influences the nutrient, depth and water content of the soil (Albaba 2004; Soromessa et al 2004). The species distribution in some sample plots of community II of the Abohoy Gara Mountain was high. Such finding can be attached to the effect of aspect and slope. By affecting the day-to-day cycle of lunar radiation, aspect influences humidity, soil moisture and air temperature of the environment (Rosenberg et al 1983). These factors might relate with distribution of the species in some sample plots of community II of study area. There was weak insignificant positive correlation between aspect and number of species (r = 0.137, P > 0.05) (Figure 4). This finding is in line with result reported from Yunmeng Mountain, China (Zhang and Shao 2015).
Shannon diversity index was negatively correlated with elevation, and their correlation was significant (r = -0.322, P < 0.05) (Figure 5A). Similarly, negative correlation between elevation and Shannon diversity index were reported in the Afro-alpine vegetation of Sanettie plateau of the Bale Mountain (Gashaw and Fetene 1996) and Abune yosef Mountain (Gebrehiwot et al 2019). Likewise, significant negative correlation (r = -0.272, P < 0.05) was occurred between elevation and species cover abundance (Figure 5B). This finding is in agreement with the report of Wondo Genet forest (Kebede et al 2013). Overall, species richness, species abundance and Shannon’s diversity get lower as elevation increases in the study area. This could be due to total atmospheric pressure, temperature and air density decrease with increasing elevation while wind velocity, radiation and precipitation increase with increasing elevation (Barry 2008).
Threats to plant resources in Abohoy Gara Mountain
According to the respondents, the highest reported threat for the plant resource in Abohoy Gara Mountain was over grazing (52), logging for construction materials (33) and agricultural expansion (22) (Table 4). Similar findings were reported elsewhere in the country (Kelbessa et al 1992; Chanie and Tesfaye, 2015; Tadesse and Teketay, 2017) in that factors such as over grazing, agricultural expansion, collection of wood for construction pose a great impact on the countries’ plant resource.
Recommended methods to conserve plant resources
The highest cited conservation approach mentioned by informants was protecting plants from livestock damage which accounted for 73 (32.6%) followed by protecting in homegarden 46 (20.54%). Furthermore, fencing 43 (19.2%) was cited to be used as a means of conserving plant resource (Supplemental material 4). Sustainable landscape use, including good rangeland management (removing livestock from stressed areas), and forest management should be maintained in Abohoy Gara Mountain so as to reduce poverty and improve ecosystem services. Species listed under IUCN Red list categories such as B. grandiflorum, I. confertiflora, E. pinifolius, S. gigas and endemic species should be prioritized for conservation. Hence, both on-site and off-site conservation is needed, particularly on-site conservation is necessary for endemic species.